Celestial happenings you can see from your own backyard

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Bouncing boulders prove Mars still rocks

Dotted streaks show the paths taken by boulders tumbling down the wall of a small crater on Mars. Click to enlarge. Credit: NASA/JPL-Caltech

Sometimes you have to stop what you’re doing and share a cool photo with your friends. Once a week I get an e-mail announcement with photo updates from the Mars Reconnaissance Orbiter (MRO), a NASA probe orbiting Mars since 2006. MRO’s main mission is to map the planet’s surface in detail with its high resolution camera; it’s also been studying Mars’ atmosphere, climate and geology.

Cropped version of the photo above with a clearer view of track details. Credit: NASA/JPL-Caltech

The photo shows something I especially love about nature – how beautiful patterns evolve by natural happenstance. Here, boulders perched in an alcove within a crater wall tumbled down the slope until coming to rest near the crater’s floor. The long, dotted tracks help us picture each bounce as gravity did its magic and teased one boulder after another down the incline.

In this even tighter crop with enhanced contrast you can see individual boulders (dark dots) that came to rest near the crater’s floor. Credit: NASA/JPL-Caltech

“Mass wasting” – the downhill movement of rocks and particles due to the force of gravity – is the technical term for what you’re seeing, and it’s as prevalent on Mars as on Earth. Landslides, avalanches and debris flows are familiar manifestations of mass wasting.

While no one’s absolutely sure what causes these slope streaks on Mars, they most likely form when fine-grained sand slides down the walls of craters. The sand lightens over time. Click to enlarge. Credit:NASA/JPL-Caltech

On Mars two of the most common forms of the process are rock landslides and dust and sand avalanches, also called “slope streaks”. Any numbers of things can trigger an avalanche. Erosion from Martian winds, subsurface water flows or vaporizing ice can steepen and widen a crater’s wall, undercutting the rim where the boulders have been in repose for millions of years. When the tipping point is reached, gravity’s tug can nudge them over the edge and down the slope.

Marsquakes and weakening of rocks from the day-night freeze-thaw cycle also play important roles in jimmying boulders loose and setting them in motion.

A small stream cuts across a pebble beach along Lake Superior north of Grand Marais, Minn. last weekend. Fast-flowing water cut the banks so steeply, pebbles tumbled into the stream and were carried away into the lake. Photo: Bob King

Last weekend I came across a delightful example of mass wasting along the north shore of Lake Superior. While crossing a pebble beach I was stopped in my tracks by a 4-foot-wide stream slicing across the strand as it hurried toward the lake. The current cut so quickly through the loose rock, pebbles along its banks tumbled continuously into the fast-moving water and were swept into the lake.

About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

Hi Carol,
There may or may not be Marsquakes today but there’s evidence from boulder trails in the Cerberus Fossae fault system that quakes may have happened in recent Martian history or may be still. While you’re right about no plate tectonics, it did have active volcanoes and active fault systems at one time in its history. Here’s an interesting article on the topic: http://www.agu.org/news/press/pr_archives/2012/2012-09.shtml